Evaluation of effect of predrying on the porous structure of water-swollen coal based on the freezing property of pore condensed water

The effect of the extent of predrying on the porous structure of water-swollen coal was examined. As-received Yallourn (YL), Beulah Zap (BZ), and Illinois #6 (IL) coals were used as the samples. They were predried at 303 K to different extents. Upon predrying, the coal samples released water in the following order: free water identical to bulk water, bound water that froze at around 226 K, and finally, nonfreezable water that never froze even at 123 K. Predried samples were swollen in water at 303 K and subjected to ¹H NMR measurements to characterize the freezing property of water retained in pores at a temperature range from 170 to 294 K. The total volume of the pores filled with water (V_p) was defined as the amount of water that was not frozen at 260 K. The removal of the nonfreezable water from YL coal by the predrying decreased the V_p of the water-swollen coal, while removal of the other types of water had little effect on V_p. Complete predrying of the other coals also reduced V_p, but to a smaller extent than for YL coal. The freezing point distribution (FPD) for pore condensed water that froze at 213-260 K was determined experimentally by NMR and also simulated numerically using a Gaussian function. A modified Gibbs-Thompson equation, which relates the freezing point depression to the pore dimensions employing a cylindrical-shaped pore model, was applied to convert FPD into pore size distribution (PSD). The PSD, expressed as pore radius, ranged from 1 to 3 nm, suggesting that the reduction of V_p for the YL coal was mainly due to the shrinkage or collapse of pores with radii around 2 nm, which are abundant in water-swollen coal before predrying.